Final answer:
Among the given atoms, rubidium requires the least amount of energy to remove its valence electron, as it lies lowest in Group 1 and has the largest atomic radius, reducing the nuclear pull on the valence electron.
Step-by-step explanation:
The atom that requires the least amount of energy to remove its valence electron in the ground state is the rubidium atom (c). Rubidium lies below lithium, sodium, and potassium in Group 1 of the periodic table. This group is characterized by having a single electron in their outermost shell. As we move down the group, the valence electron is farther from the nucleus due to the increasing number of electron shells.
This increases the atomic radius and reduces the effective nuclear charge felt by the valence electron, thus requiring less energy to remove it. Therefore, among the options given, rubidium, being the lowest in the group, will require the least energy to remove its valence electron.